1. Field of the Invention
The invention relates generally to a method and an apparatus for forming glass sheet, and more particularly to drawing glass sheet from a glass preform.
2. Technical Background
There is an increasing demand for flat glass sheets, especially precision flat glass sheets, i.e. of high surface quality and consistent thickness, which are made of a glass-based material such as a special glass or a glass-ceramic. Flat panel displays incorporating such flat sheets have received a great deal of attention recently. Much of the attention has centered on small units such as those used in laptop computers. However, consideration is now being given to larger units for information and entertainment applications, where there is a surge of interest in glasses having a high strain point in the display industry. These glasses are needed to make flat display panels for the next-generation liquid crystal displays (LCDs), e.g., active matrix LCDs (AMLCDs), and other advanced displays, e.g., plasma displays. Generally speaking, a strain point of at least 700° C. is desired. Preferably, the strain point is greater than 800° C. In the case of AMLCDs, the need for such a high strain point is dictated by the interest in bonding silicon chips or arrays directly onto glass substrates. Fabrication of poly-silicon on glass substrates is further facilitated by process temperatures of 900° C. or greater. In order to accomplish this objective, it is necessary for the thermal expansion behavior of the glass to be very similar to that of silicon, and for the strain point of the glass to be high enough so that compaction (also known as shrinkage or densification) and/or warping of the glass does not occur after the silicon chips are bonded to the glass and the glass is subsequently heated in further processing steps.
Liquid crystal displays (LCDs) may take the form of two basic matrix types, intrinsic or extrinsic matrix addressed. The intrinsic matrix type relies upon the threshold properties of the liquid crystal material. The extrinsic, or active matrix (AM), type has an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs), that supplies an electronic switch to each pixel.
In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5–10 μm. The glass sheets must be transparent, and must withstand the chemical conditions to which they are exposed during display processing. Otherwise, the needs of the two matrix types differ.
Intrinsically addressed LCDs are fabricated using thin film deposition techniques at temperatures ≦350° C., followed by photolithographic patterning. As a result, the substrate requirements therefore are often the same as those for segmented displays. Soda-lime-silica glass with a barrier layer has proven to be adequate for most needs.
A high performance version of intrinsically addressed LCDs, the super twisted nematic (STN) type, has an added requirement of extremely precise flatness for the purpose of holding the gap dimensions uniform. Because of that requirement, soda-lime-silica glass used for those displays must be polished. Alternatively, a precision formed, barium aluminoborosilicate glass, marketed by Corning Incorporated, Corning, N.Y. as Code 1737, may be used without polishing.
Extrinsically addressed LCDs can be further subdivided into two categories; viz., one based on MIM or amorphous silicon (a-Si) devices, and the other based on polycrystalline silicon (poly-Si) devices. The substrate requirements of the MIM or a-Si type are similar to the STN application.
Devices formed from poly-Si, however, are processed at higher temperatures than those that are employed with a-Si TFTs. Substrates capable of use temperatures (taken to be 25° C. below the strain point of the glass) of 600° C.–800° C. are demanded, although the actual temperature required is mandated by the particular process utilized in fabricating the TFTs. Those TFTs with deposited gate dielectrics require temperatures in the range of 600° C.–650° C., while those with thermal oxides call for temperatures of about 800° C.
Both a-Si and poly-Si processes demand precise alignment of successive photolithographic patterns, thereby necessitating that the thermal shrinkage of the substrate be kept low. The higher temperatures required for poly-Si mandate the use of glasses exhibiting higher strain points than soda-lime-silica glass in order to avoid thermal deformation (compaction, i.e. shrinkage) of the sheet during processing. As can be appreciated, the lower the strain point, the greater this dimensional change. Thus, there has been considerable research to develop glasses demonstrating high strain points so that compaction is minimized during device processing at temperatures greater than 600° C.
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four properties which have been deemed mandatory for a glass to exhibit in order to fully satisfy the needs of a substrate for LCDs:
First, the glass must be essentially free of intentionally added alkali metal oxide to avoid the possibility that alkali metal from the substrate can migrate into the transistor matrix;
Second, the glass substrate must be sufficiently chemically durable to withstand the reagents used in the TFT matrix deposition process;
Third, the expansion mismatch between the glass and the silicon present in the TFT array must be maintained at a relatively low level even as processing temperatures for the substrates increase; and
Fourth, the glass must be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.
That last requirement is most difficult to achieve inasmuch as it demands a sheet glass production process capable of producing essentially finished glass sheet.
The two methods commonly used in manufacturing LCD substrates are the float process and the fusion process. Both of these processes require a refractory glass melter to deliver a stream of glass to a sheet-forming device. In the case of high strain-point glass compositions, a relatively large high-temperature glass melter is needed to deliver a high-quality stream of glass to the sheet-forming device. This is because high strain-point glasses have high fusion temperatures, typically in excess of 1700° C.
In the float process, a stream of molten glass is discharged from a melting furnace into a float furnace that contains a liquid metal medium. Typically, the metal is tin. The atmosphere in the float furnace is controlled to prevent oxidation of the tin. The molten glass floats and spreads out on the liquid tin in the form of a flat, continuous ribbon. The ribbon of glass is conveyed into an annealing lehr or cooling tunnel, where it is cooled at a controlled rate to ambient temperature. The cooled glass has a flat, smooth surface that requires a minimum of further finishing by processes such as grinding and polishing.
However, it is very difficult to form glasses having high strain points in an enclosure containing molten tin. This is because tin has high vapor pressures at temperatures in excess of 1050 to 1100° C. At the high forming temperatures required for high strain-point glasses, the molten tin will vaporize inside the float furnace and subsequently condense in colder parts of the furnace. In some cases, the condensation may be sufficiently high to create what is referred to as “tin rain,” a situation where tin rains on the glass and is incorporated on the glass surface.
In the fusion process, a glass-forming melt flows into a refractory trough and then overflows in a controlled manner from either side of the trough. A key advantage of this process is that the surface of the glass sheet which is ultimately formed does not come in contact with any refractory material or other forming equipment. Another benefit of the process is that it yields a very flat and uniformly thick sheet of glass. As a result, no secondary processing is needed to obtain a smooth, flat, and uniform sheet of glass for display applications. However, the method suffers from not being able to process glasses having high strain points due to the high temperatures required, since such temperatures greatly accelerate deterioration of the glass forming components, and the potential for increased contamination of the glass melt. Typically, it is desirable to form the glass at viscosities in the range of 105 to 106 poise to obtain optimum flatness and uniform thickness.
A brief description of both the fusion draw and float processes are given in a manuscript entitled “Glass” by D.C. Boyd and D. A. Thompson, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, pp. 807–880 (see pages 860–863). The fusion draw process is also described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both issued to Dockerty. Unfortunately, neither the fusion draw process nor the float glass process is effective in producing flat sheet from a glass composition having a high strain point, one whose strain point may, for example, exceed 900° C.